What is the payback period?

The payback period is the time at which the entire investment from year 0 has been recouped. This is interpolated from the accumulated cash flow. The cash flow is calculated individually for each year; in year 0, only the initial investment (minus any investment subsidies) forms the cash flow, which is therefore negative. In all subsequent years, the cash flow is calculated from income and expenditure for feed-in/purchase of electricity compared to the situation without a PV system, i.e., the simulation is carried out twice, once with and once without a PV system. These revenues and expenses are calculated considering the inflation of the feed-in tariff, grid costs and purchase tariffs. In addition, the maintenance costs for the PV system and possibly the battery are considered. (The difference in the cash flows between these two simulations is calculated individually for each year and denotes the cash flow in year x. The cash flow is then calculated cumulatively. The cash flow is then cumulated. The payback period is linearly interpolated between the two crucial years (year x where the cumulative cash flow is negative for the last time and year x where the cash flow is positive for the first time). If the cash flow remains negative over the entire 25 years of the simulation, the last two years are used for the interpolation.